Multi-gauge strips are mainly used as materials for terminals of lead frames, relays, connectors, etc. of power trasistors. The present invention relates to a method of producing a multi-gauge strips by means of forming copper, a copper alloy, or a metal strip which can undergo plastic work in such a manner that its two sides in the widthwise direction are thinner than the center portion.
As for the conventional methods of producing a multi-gauge strips, the following methods are generally known: a scalping method of thinly cutting the surfaces of the two sides of a strip to reduce the thickness thereof while leaving its center portion intact by means of using a cutting tool; a welding method of joining thin materials to the two sides from the center of a strip; a seesaw method (hereinafter SS) of rolling at a right angle to the direction of progression of a material in feed; and a rectilinear rolling method (hereinafter RL) of rolling in the direction of progression of a material in feed. The items of specification requirements for a multi-gauge strips are as follows: shape, thickness, precision, surface roughness, straightness, hardness, etc. In this regard, with the scalping method, among other methods aforementioned, the quality of the material at the thick portion and that of the thin portion are the same as a result of scalping the surfaces of the two sides of the same material, and thus the method is able to satisfy the hardness requirement among others, but it nonetheless leads to difficulties in satisfying other specification requirements. Moreover, in conjunction with the losses associated with the cut portions, the size of the material is set to be larger than that of the finished product, and these factors lead to a disadvantage of lowering the utility ratio of a material.
With respect to the welding method, as well, even if the welding can be achieved in an instant, a source of heat of high temperature is required for such welding. To void interposition of oxides at the welding area on account of such welding, it needs a non-oxide atmosphere in a nearly vacuum condition, and to maintain such a non-oxide atmosphere, it necessarily incurs high installment and operation costs, which in turn leads to a high production cost. Moreover, with respect to the welding area, the quality reliability is lowered, which in than tends to limit the utility of a strip.
As for another method in addition to those aforementioned, there is a seesaw method (SS) or a V-MILL method, wherein pressure is put onto a strip material (M) which in turn is placed on a metal mold (TL), as shown in FIG. 1, with a direction of rolling in a back-and-forth motion, or by a method of working from left to right, much like a seesaw, at a right angle (RA) to the material (M). In this manner, the center portion of the strip material (M) is indented by the lower surface (S) of the mold, and the two sides thereof in the width direction are stretched out to the respective space (S) at either side, thereby forming a thick portion (TK) and a thin portion (TN) therein. In this method, pressure is effectuated vertically on the material, which in turn is placed on the flat metal mold. As such, the stretching work on the material is made difficult due to a sticking caused by friction between the mold and the material. As compare to drawing works, it requires pressure of several tens of times, which in turn necessitates equipment of large capacity in addition to the high-strength metal molds. Moreover, since the operation of a rolling roll or a metal mold must be carried out in the direction which is left and right to the direction of progression of the material, there is an ensuing problem of requiring high-priced special-purpose equipment of large capacity, instead of ordinary general-purpose equipment. Further, since the ends of the two sides of a material in the width direction must be maintained at a slant while stretching the same for continuous formation, it limits the feed speed to the level of 1xcx9c2 m/minute, which in turn dramatically lowers productivity.
Further, as another method in addition to those aforementioned, the rectilinear rolling method (RL) involves passing the material (M) between the two rolling rolls (R), one on the top and the other on the bottom, each having a rectilinear indentation and a protrusion. In this manner, by folding the two sides of the material in the widthwise direction (WO), the primary rolling (RL1) is carried out into the shape having a protrusion at the center portion (V1) thereof Then, in the secondary rolling (RL2), by compressing the upper end of the center portion (V1), the widths (L1, V1, W1) and the thickness (T1, t1) formed during the primary rolling (RL1) are formed into the following widths (L2, V2, W2) and thicknesses (T2, t2). Receiving the material therefrom, the tertiary rolling (RL3) using the same method of the primary rolling (RL1) carries out the process of rolling it into the shape with the following widths (L3, V3, W3) and the thicknesses (T3, t3). By repetitively carrying out the aforementioned process, the material is formed into the following specifications of widths (L, V, W) and thicknesses (T, t).
Here, the width of the center portion is reduced from V1 to V2 and then to V3, and the width of the either side is widened from L1 to L2 and then to L3. The total width thereof is enlarged from W1 to W2 and then to W3, and the thickness at the center is reduced from T1 to T1 and then to T3. The thickness of the either side from the center portion (V1) is also reduced from t1, to t2 and then to t3. In this manner, a multi-gauge strips is formed thereby, the art of which is generally known in the field.
However, in the aforementioned rectilinear rolling method, due to the peculiarity of the rolling work, the direction of staining (stretching) of the material is such that the staining of the material is achieved only in the lengthwise direction of the material, which is the direction of progression of rolling. The straining of the material in the width direction, which is perpendicular to the direction of progression of the material, or the elongation as a result of stretching, is insignificant as compared to that in the direction of progression of the material. As such, by compressing the lower side of said center portion (V1) in the width direction, the ends of the two sides are widened while the thickness (t1) of the either side is reduced to the thickness (t2) by means of rolling work. As such, 8 to 10 units of rolling mills must be arranged continuously therein since the above process of widening by stretching the two sides (L2) in the widthwise direction must be carried out repetitively for about ten times or so. Hence, in terms of facility investment, there is indeed a problem of economical efficiency.
Moreover, as for the tandem rolling method having a continuous placement of rolling rolls in a serial manner, the continuous rolling is carried out in such a manner that a strip with a given length after undergone the rolling process in the preceding roll (R) is received by the subsequent roll (R), which in turn is received by the next subsequent roll (R).
With respect to the length of the material, stretched at that time, the length of the strip is elongated in the order of primary, secondary and tertiary rollings. As such, with the same rotation speed with resect to the primary, secondary and tertiary rolling rolls, the continuous work cannot possibly be carried out therein. To solve this problem, equipment with a special function to control the rotation speed of the rolls (R) is required, which is rather problematic in itself.
Moreover, the rolls (R), which are used in the rolling process are tools with specifications of multi-gauge strips materials (P), and the rolls (R) of various specifications in small lots must be provided beforehand, which leads to a problem of rising cost with the inconvenience of operational management in disassembling and changing the rolls (R) at the time of working with a respective specification.
In order to solve the aforementioned problems of the prior art, the present invention presents a method producing a multi-gauge strips, which comprises as follows: forming a flat strip into a semi-circular shape in the widthwise direction by using an ordinary general-purpose press; reducing the thickness of the two sides of the semi-circular shape thereof, forming into a xe2x80x9cUxe2x80x9d shape by stretching in length the ends of the two sides of the semi-circular shape thereof with a reduced thickness, and then forming a protrusion of a curved surface along the inner circumference of the xe2x80x9cUxe2x80x9d shape thereof, and forming the xe2x80x9cUxe2x80x9d shape therein with a protrusion of a curved surface along the inner circumference thereof into a flat multi-gauge strips, wherein said protrusion of a curved surface along the inner circumference is continuously formed into a thick portion at the center of said flat multi-gauge strips, and the two sides of the xe2x80x9cUxe2x80x9d shape thereof into thin portions at either side of said flat multi-gauge strips.
In order to achieve the technical objective therein, the present invention provides a method of producing a multi-gauge strips, in relation to a method of producing a multi-gauge strips, which includes the steps of forming a flat strip material into semi-circular and xe2x80x9cUxe2x80x9d shapes by means of press work while sequentially feeding said strip material between an upper mold and a lower mold, wherein said method comprises the steps of: (a) forming a flat strip material into a semi-circular shape in the widthwise direction; (b) gradually forming the semi-circular shape thereof as formed above into a xe2x80x9cUxe2x80x9d shape, in which the ends of the two sides thereof are connected by a slanted side, and at the same time press-forming a protrusion of a curved surface, slanted gradually, on the inner circumference of the semi-circular and xe2x80x9cUxe2x80x9d shapes thereof, (c) forming said slanted side into a straight line to form the material into the xe2x80x9cUxe2x80x9d shape while forming the slanted protrusion of a curved surface into a protrusion of a curved surface having the same amount of protrusion; (d) forming the material into a strip with curvature by forming said protrusion of a curved surface into a flat protrusion; (e) forming the material into a flat multi-gauge strips by flat-pressing the two sides and the flat protrusion of said strip with curvature, wherein the protrusion and the two sides thereof are set to in parallel thereby; and (f) causing the ends of the two sides of said flat multi-gauge strips.